Absolute encoder utilizing light of different wavelengths to reduce cross-talk

ABSTRACT

An encoder having a code scale, an illumination system, and a plurality of photodetectors is disclosed. The code scale has a plurality of tracks that are illuminated by the illumination system. Each photodetector receives light from a corresponding one of the tracks and generates a signal indicative of a quantity of light received from that track. Each photodetector is limited to detecting light in a band of wavelengths corresponding to that track. Each track generates light in a band of wavelengths corresponding to that track, and the band of wavelengths corresponding to one of the tracks is different from the band of wavelengths corresponding to the tracks that are next to that track. The tracks can include alternating reflective and absorptive stripes, alternating transmissive and reflective stripes, or alternating luminescent and absorptive stripes.

BACKGROUND OF THE INVENTION

Encoders provide a measurement of the position of a component in asystem relative to some predetermined reference point. Encoders aretypically used to provide a closed-loop feedback system to a motor orother actuator. For example, a shaft encoder outputs a digital signalthat indicates the position of the rotating shaft relative to some knownreference position that is not moving. A linear encoder measures thedistance between the present position of a moveable carriage and areference position that is fixed with respect to the moveable carriageas the moveable carriage moves along a predetermined path.

To measure the position of a first component that moves with referenceto a second component, an encoder typically uses one or more tracks inwhich each track consists of a series of alternating dark and lightstripes that are viewed by a detector that outputs a digital valuedepending on whether the stripe currently being viewed is light or dark.The track is affixed to one of the components and the detector isaffixed to the other.

Encoders can be divided into two broad classes. An incremental encodertypically utilizes a single track that is viewed by a detector thatdetermines the direction and the number of stripes that pass by thedetector. The position is determined by incrementing and decrementing acounter as each stripe passes the detector. The counter is reset when areference mark is detected. An absolute shaft encoder typically utilizesa plurality of tracks. An N-bit binary encoder typically utilizes N suchtracks, one per bit.

While incremental encoders are less expensive than absolute encoders,incremental encoders are subject to errors that are often unacceptable.For example, if the circuitry fails to count a transition from a lightto dark stripe, the counter, and hence, the position measurement will bein error until the counter is reset the next time the reference point isdetected. Absolute encoders, in contrast, can be in error for at mostone stripe of the track having the finest resolution. Hence, absoluteencoders are preferred in many applications in spite of the additionalcost associated with such encoders.

As the size of the mechanical systems that utilize encoders decreases,the size of the encoders must also decrease. One factor that limits theminimum size of an absolute encoder is cross-talk between the detectorsused on the various tracks. Each track in the encoder is illuminatedwith a light source. The light from the illuminated track is imaged ontoa corresponding photodetector that determines whether the stripecurrently being viewed is light or dark. The light that strikes thedetector consists of light that is reflected from the code stripes ofthe track corresponding to that detector as well as light from anadjacent track that is scattered into the detector due to imperfectionsin the optical system and code stripes. This scattered light forms abackground that reduces the signal-to-noise ratio of the detector, andhence, can lead to errors in the measured position. As the code stripesare reduced in size in an effort to reduce the size of the encoder, thelight available from a track decreases, since the size of the stripesmust be reduced. In addition, the distance between the tracks decreases,which, in turn, reduces the buffer space between the tracks thatprotects each detector from scattered light from a neighboring track.Both of these factors lead to reduced signal-to-noise ratios.

The cross-talk problem is particularly acute in reflective encoders. Ina reflective encoder, each track consists of a series of reflective andabsorptive stripes. Light is reflected from the reflective stripes intothe detector associated with the track. While the absorptive stripes canbe made nearly ideal by utilizing a hole in the code scale for theabsorptive stripes, the reflective stripes are less than ideal. Ideally,the reflective stripes are perfect mirrors. However, in practice, themirrors have imperfections. In addition, debris accumulates on thesurface over time. These factors result in a surface that scatters someportion of the light incident on the surface. Some of the scatteredlight falls on the detectors corresponding to the adjacent tracks.

SUMMARY OF THE INVENTION

The present invention includes an encoder having a code scale, anillumination system, and a plurality of photodetectors. The code scalehas a plurality of tracks that are illuminated by the illuminationsystem. Each photodetector receives light from a corresponding one ofthe tracks and generates a signal indicative of a quantity of lightreceived from that track. Each photodetector is limited to detectinglight in a band of wavelengths corresponding to that track. Each trackgenerates light in a band of wavelengths corresponding to that track,and the band of wavelengths corresponding to one of the tracks isdifferent from the band of wavelengths corresponding to the tracks thatare next to that track. The tracks can include alternating reflectiveand absorptive stripes, alternating transmissive and reflective stripes,or alternating luminescent and absorptive stripes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a transmissive encoder.

FIG. 2 illustrates one type of reflective encoder.

FIG. 3A is a top view of shaft encoder according to one embodiment ofthe present invention.

FIG. 3B is a top view of a shaft encoder according to another embodimentof the present invention.

FIG. 4 is a cross-sectional view of shaft encoder 30 through line 4-4shown in FIG. 3.

FIG. 5 is a top view of shaft encoder 60.

FIG. 6 is a cross-sectional view of shaft encoder 60 through line 6-6shown in FIG. 5.

FIG. 7 is a cross-sectional view of a multi-channel transmissive encoder100 according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Refer now to FIGS. 1-3, which illustrate some typical encoder designs.The encoder can be divided into an emitter/detector module 15 and a codewheel or code strip. To simplify the terminology used herein, the term“code scale” is defined to include both linear code strips and circularcode wheels. Module 15 includes an emitter 11 that illuminates a portionof the code scale 12. A detector 13 views the illuminated code scale.The emitter typically utilizes an LED as the light source. The detectoris typically based on one or more photodiodes. FIG. 1 illustrates atransmissive encoder. In transmissive encoders, the light from theemitter is collimated into a parallel beam by a collimating optic suchas lens 24. Code scale 12 includes opaque stripes 16 and transparentstripes 17. When code scale 12 moves between emitter 11 and detector 13,the light beam is interrupted by the opaque stripes on the code scale.The photodiodes in the detector receive flashes of light. The resultantsignal is then used to generate a logic signal that transitions betweenlogical one and logical zero.

The detector can include an imaging lens 25 that images the collimatedlight onto the photodiode. Lens 25 can be used to adjust the size of thelight stripes to match the size of the photodiode or photodiodes in thedetector. When used in this manner, the photodetector is placed at apoint between the code scale and the focal point of lens 25. Thedistance between the photodetector and the lens determines the size ofthe code scale image on the photodetector.

In general, the collimator is constructed from two separate sub-modulesthat are provided to the manufacturer of the encoder. The firstsub-module includes the light source consisting of emitter 11 and lens24. The second sub-module consists of photodetector 13 and lens 25. Thecode scale consists of either absorptive stripes or holes. Stray lightdirected toward a neighboring track's detector could be generated bylight reflecting off of the side of the holes.

FIG. 2 illustrates one type of reflective encoder. In reflectiveencoders, the code scale includes reflective stripes 18 and absorptivestripes 19. The light from the emitter is reflected or absorbed by thestripes on the code scale. The emitter includes an optical system suchas a lens 21 that images the emitter light source into the detector whenthe light strikes a reflective stripe on the code scale. The output fromthe photodetector is again converted to a logic signal. In embodimentsin which the photodetector includes a plurality of photodiodes thatprovide a signal that depends on matching an image of the strips to thephotodiodes, a second lens 27 can be included to adjust the size of thecode scale image to the size of the photodetectors in a manner analogousto that described above. This arrangement assumes that the reflectivestripes are perfect mirrors. As noted above, the mirrors that can beconstructed at a reasonable cost are less than perfect, and hence, straylight that can reach neighboring detectors is generated.

Refer now to FIGS. 3A and 4, which illustrate a reflective shaft encoderaccording to one embodiment of the present invention. FIG. 3 is a topview of shaft encoder 30, and FIG. 4 is a cross-sectional view of shaftencoder 30 through line 4-4 shown in FIG. 3A. Shaft encoder 30 includesa code scale having a number of concentric tracks. To simplify thedrawing only the three outer tracks 54-56 are shown in the drawing. Eachtrack has alternating reflective and absorptive stripes. Exemplaryreflective and absorptive stripes are shown at 32 and 33, respectively.Code scale 31 rotates about a shaft 34.

Shaft encoder 30 includes one emitter detector module for each track.The emitter detector modules corresponding to tracks 54-56 are shown at41-43, respectively. Each emitter detector module includes a lightsource and a photodetector. To simplify the drawings, any lensesassociated with the light source or photodetector have been omitted fromthe drawing; however, it is to be understood that light sources andphotodetectors may include one or more lenses. The light sourcescorresponding to emitter detector modules 41-43 are shown at 44, 47, and51, respectively. The photodetectors corresponding to emitter detectormodules 41-43 are shown at 45, 48, and 52, respectively.

The light sources utilized in shaft encoder 30 emit light in differentbands. The bands are chosen such that adjacent emitter detector modulesutilize different wavelengths. Hence, light source 47 emits light at adifferent wavelength than light sources 44 and 51. The light sourcecorresponding to each emitter detector module can have a uniquewavelength or band of wavelengths; however, as long as the light sourcesadjacent to any given light source emit light at a different wavelength,a significant improvement in the cross-talk problem discussed above canbe achieved.

In shaft encoder 30, each photodetector includes a band-pass filter thatlimits that photodetector's response to light emitted by the lightsource in that photodetectors emitter detector module while blockinglight of the wavelengths generated by the light sources in the adjacentemitter detector modules. The bandpass filters corresponding to emitterdetector modules 41-43 are shown at 46, 49, and 53, respectively. Hence,any stray light from emitter detector module 42 that reaches emitterdetector module 41 or emitter detector module 43 will be eliminated bybandpass filters 46 and 53.

In the embodiment shown in FIG. 3A, the emitter detector modules arealigned radially such that each emitter and detector lies on a radius ofthe code scale. However, other arrangements can be utilized. Refer nowto FIG. 3B which is a top view of a shaft encoder utilizing a differentemitter detector module orientation. The emitter detector modules inshaft encoder 30′ are oriented such that the line connecting the emitterand detector is tangential to a circle centered on shaft 34.

The above-described embodiment of the present invention is directed to amulti-track encoder in which the reflective areas of the code scale actas mirrors that, together with the various lenses in the light sourceand photodetector, image the light source onto the photodetector.However, the present invention can also be used to reduce cross-talk inimaging detectors in which the code scale is illuminated with a diffuselight source and the lenses in the photodetectors image a portion of thecode scale onto one or more photodiodes in the photodetector.

Refer now to FIGS. 5 and 6, which illustrate an imaging reflectiveencoder according to another embodiment of the present invention. Shaftencoder 60 measures the rotation of a shaft 64. FIG. 5 is a top view ofshaft encoder 60, and FIG. 6 is a cross-sectional view of shaft encoder60 through line 6-6 shown in FIG. 5. Shaft encoder 60 also includes acode scale 61 having a number of concentric tracks. To simplify thedrawing only the three outer tracks 81-83 are shown in the drawing. Eachtrack has alternating bright and absorptive stripes. Exemplary brightand absorptive stripes are shown at 62 and 63, respectively. In contrastto the mirror-like stripes discussed above, the bright stripes form adiffuse light source when illuminated.

The underside of the code scale is illuminated by a light source 74.Light source 74 can be a diffuse light source. Alternatively, lightsource 74 can be constructed from a point source and a lens thatproduces a light source with sufficient spread in output angles toilluminate the underside of the code scale in the area viewed by thephotodetectors associated with the various tracks.

Each of the tracks has a corresponding detector. The detectorsassociated with tracks 81-83 are shown at 71-73, respectively. In theembodiment shown in FIGS. 5 and 6, each detector includes aphotodetector, a bandpass filter, and a lens. The photodetectorscorresponding to detectors 71-73 are shown at 91-93. The bandpassfilters corresponding to detectors 71-73 are shown at 94-96,respectively, and the imaging lenses corresponding to detectors 71-73are shown at 97-99, respectively.

Each imaging lens images a portion of the code scale corresponding toone of the tracks onto the photodetector corresponding to that track.The photodetector typically includes one or more photodiodes. The numberof photodiodes and the arrangement of those photodiodes depends on thespecific detection algorithm implemented in the detector. Since thepresent invention does not rely on any specific detection algorithm, thedetails of the specific arrangement of photodiodes will not be givenhere.

Each track has an associated “color”. That is, the light leaving thebright stripes on the track is restricted to a band of wavelengths. Thebandpass filter in the detector associated with that track passes thatband of wavelengths while blocking light in the bands generated by thebright stripes on the adjacent tracks. Hence, cross talk is reduced in amanner analogous to that discussed above with reference to shaft encoder30. The specific color emitted by the bright stripes on each track canbe controlled by providing a white code scale covered with a bandpassfilter. Note that the bandpass filter can be a layer that is depositedover the track on the code scale. In this case, light source 74 mustemit light in all of the wavelength bands used by the tracks. That is,light source 74 must be a broadband source.

While the embodiment shown in FIGS. 5 and 6 utilizes a particularlocation for light source 74, other placements could be used. Lightsource 74 can be placed at any location that provides sufficientillumination of the code scale. Furthermore, multiple light sources atdifferent locations could also be utilized.

Alternatively, each bright stripe on a given track can be coated with afluorescent material that emits light in the wavelength band associatedwith that track. In this case, light source 74 can be a monochromaticsource that emits light at a wavelength that excites all of thephosphors on the code scale. For example, light source 74 could use ablue or UV emitting LED.

Encoder 60 utilizes detectors in which each detector has a separateimaging lens. This arrangement is adapted to a mass produced detectionmodule in which the photodetector and the imaging lens are prepackaged.The assembler of the part using the encoder then positions theindividual detector modules under the corresponding tracks and appliesthe appropriate bandpass filters. However, embodiments in which theindividual lenses are replaced with a single imaging lens that imagesthe relevant region of the code scale onto all of the detectors can alsobe constructed without departing from the teachings of the presentinvention.

The above-described embodiments of the present invention have beendirected to reflective encoders. However, the same principles can beapplied to transmissive encoders. Refer now to FIG. 7, which is across-sectional view of a multi-channel transmissive encoder 100according to one embodiment of the present invention. To simplify thedrawing, only three channels 101-103 are shown. Code scale 104 includesthree tracks corresponding to channels 101-103. Each track includesalternating transmissive and opaque stripes.

Each track is illuminated by a light source on one side of the codescale. The light sources corresponding to tracks 101-103 are shown at111-113, respectively. Each light source generates a collimated beam oflight that is modulated by the stripes on the code scale. Each lightsource typically utilizes an LED and a collimating lens. For example,light source 111 utilizes LED 121 and lens 131. In this embodiment, thelight sources generate light beams having different colors such thatadjacent tracks are illuminated with light from different regions of thespectrum.

The light passing through code scale 104 from each light source ismeasured by a corresponding detector. The detectors corresponding totracks 101-103 are shown at 141-143, respectively. In this embodiment,each detector includes a lens for imaging the collimated light onto aphotodetector and a bandpass filter that limits the light reaching thephotodetector to light within a predetermined band corresponding to thatdetector. The photodetector, bandpass filter, and lens corresponding todetector 141 are shown at 151, 161, and 171, respectively. However,embodiments in which the imaging functions are provided solely in thelight source could also be constructed. In such embodiments, thedetectors do not require lenses.

It should be noted that embodiments in which the individual lightsources are replaced by a single collimated broadband source could alsobe constructed. In this case, transmissive stripes can include abandpass filter to pick out the desired portion of the spectrum.Alternatively, an additional band pass filter can be placed between thelight source and the corresponding track so as to select the desiredportion of the spectrum.

The above-described embodiments of the present invention have beendirected to circular encoders of the type used to encode the motion of ashaft. However, linear encoders utilizing the same principles can alsobe constructed.

It should also be noted that the present invention is not limited toabsolute encoders. The present invention can be utilized to constructany multi-track encoder.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

1. An encoder comprising: a code scale having a plurality of tracksthereon, wherein each track comprises a plurality of alternatingluminescent and absorptive stripes, wherein the luminescent stripes ofeach track generate light in a unique band of wavelengths correspondingto that track, wherein the unique band of wavelengths corresponding tothat track is different from another band of wavelengths correspondingto an adjacent track; an illumination system to illuminate the tracks,wherein said illumination system emits light to excite the luminescentstripes of a particular track which emit light in a band of wavelengthscorresponding to the particular track; and a plurality ofphotodetectors, each photodetector to receive light from a correspondingone of the tracks and to generate a signal indicative of a quantity oflight received from that track, each photodetector being limited todetecting light in a band of wavelengths corresponding to that track. 2.The encoder of claim 1, wherein the luminescent stripes are coated witha fluorescent material that emits light, in response to incidentillumination, in the band of wavelengths corresponding to that track. 3.The encoder of claim 2, wherein the illumination system furthercomprises a monochromatic light source to emit light at a wavelengththat excites phosphors on the luminescent stripes of the code scale. 4.The encoder of claim 2, wherein the illumination system furthercomprises a light source to generate light in multiple bands ofwavelengths.
 5. The encoder of claim 4, wherein one of saidphotodetectors comprises a photodiode and a bandpass filter for limitinglight reaching said photodiode to light in the band of wavelengthscorresponding to that photodetector.